In studying the correlation between the amount of time children watch violence on television and children's levels of aggressiveness, researchers found that a lack of adult supervision could be a third variable. To control for the third variable, researchers observed children who experienced different amounts of supervision. They made sure that for every child observed who watched a lot of violence on television and was supervised X% of the time, they also observed another child who did not watch a lot of violence on television and was supervised X% of the time. This method is called: